In accordance with enlargement of use of a lithium ion secondary battery, there is a need to allow lithium ion secondary battery to have high capacity and a large area, but it is difficult to secure safety of the battery due to an increase in energy accumulated in the battery caused by an increase in capacity of the battery. It is essential to secure safety of a battery in accordance with the increase in capacity of a lithium second battery. Recently, research for improving safety of the battery has been significantly spotlighted.
In order to secure and improve safety of the lithium secondary battery, physical and electrochemical safety of a separator among various components contained in the lithium secondary battery is particularly important. As a generally used separator for a lithium secondary battery, a polyolefin based microporous thin film made of a polyethylene or polypropylene material has been used. In the case of the polyolefin based separator as described above, thermal safety of a main material forming the separator itself is not high, such that a microporous film may be easily damaged or deformed by an increase in temperature generated by an abnormal behavior of the battery. In addition, a short-circuit may occur between electrodes due to a damage or deformation of the microporous film, and there is a risk of over-heating, igniting or an explosion of the battery. Recently, a number of battery ignition or explosion cases have been reported, and these cases are due to lack of securing safety which should be essentially accompanied depending on an increase in capacity of the battery. Therefore, there is a need to develop a battery of which safety is improved in addition to high capacity, and in order to develop this battery, research into a technology capable of improving safety in accordance with high capacity of the battery by imparting a novel separate property to a separator has been conducted.
As the easiest method for improving safety of the battery, there is a method of increasing a thickness of a separator. However, in this method, mechanical strength, or the like, may be improved depending on an increase in thickness of the separator, but capacity and output of the battery are deteriorated, and this method is not suitable for miniaturization of the battery.
As one of methods for improving safety of the battery, a method of improving safety of the battery by forming a coating layer containing an inorganic material on a polyolefin based film to improve thermal safety of a separator has been used. An example of this method, a separator in which an active layer coated with a mixture of inorganic particles and a polymer binder is formed on a polyolefin microporous film has been disclosed in Korean Patent No. 0775310. In addition, a separator of which thermal safety is improved by forming a coating layer containing inorganic particles on non-woven fabric without using a polyolefin based porous film has been developed.
However, these separators may be unsuitable for being used to secure safety of a recent battery of which capacity has been increased in view of thermal and electrochemical safety of the separator. Particularly, in the case of using the non-woven fabric, the separator may be damaged during a process of assembling the battery due to low strength of the non-woven fabric itself, and may also be damaged in the battery even by weak impact applied to the battery, such that the non-woven fabric is unsuitable for securing safety.